1. Field of the Invention
This invention relates generally to microwave-frequency oscillators, and relates more particularly to circuitry adapted for switching, attenuating, modulating, or otherwise controlling the output amplitude of microwave-frequency oscillators.
2. Description of the Relevant Art
A typical microwave oscillator includes an active device capable of oscillating at microwave-range frequencies plus associated oscillation-tuning circuitry capable stabilizing the oscillator at a particular frequency. Many microwave-frequency oscillators utilize a bipolar or field-effect transistor as the active device, in which case the oscillator includes bias circuitry that establishes the bias conditions of the transistor and reactive-feedback circuitry that creates, in conjunction with the transistor, a negative resistance that allows the transistor to oscillate. So long as the negative resistance is sufficient, the transistor will oscillate at a frequency dictated by the oscillation-tuning circuitry.
In order to switch some prior art oscillators on and off, common practice was to turn on and off, respectively, the bias circuit. Switching the oscillator in this fashion creates frequency stability problems due to thermal transients related to the transistor cooling when the bias voltage is removed and warming when the bias voltage is reapplied. Alternatively, a shunting circuit attached between the oscillation-tuning circuitry and the transistor could be selectively activated in order to cancel the oscillation condition by perturbing the impedance of the oscillation-tuning circuit.